package main

import (
	"fmt"
)

func permutation(s string) (res []string) {
	trace , book, n := "" , make(map[int]int), len(s)
	set := make(map[string]int)
	var depth func()
	depth = func() {
		if _ , ok := set[trace] ;len(trace) == n && !ok {
			res = append(res, trace)
			set[trace] ++
			return
		}
		for i := 0 ; i < n ; i++ {
			if v , _ := book[i] ; v > 0    {
				continue
			}
			book[i] ++
			trace += string(s[i])
			depth()
			book[i] --
			trace = trace[:len(trace)-1]
		}
	}
	depth()
	return res
}

func main() {
	fmt.Println(permutation("aab"))
}